1. Field of the Invention
The present invention relates generally to systems which monitor the subsidence and compaction of subterranean formations by measuring the shifting of spaced-apart radioactive marker bullets implanted in the formation. In another aspect, the invention concerns a method and apparatus for retrieving stray radioactive marker bullets loosely positioned in a subterranean borehole.
2. Description of the Prior Art
The production of water and hydrocarbon fluids from subterranean formations depletes reservoir pressure and removes the fluids from the interstatial pores of the subterranean formation. As fluids are removed, the reduction of pressure in the subterranean formation may cause collapsing (i.e., subsidence) and compaction of the subterranean formation. This subsidence and corresponding compaction is heightened when the subterranean formation comprises chalk or other rock having relatively high porosity and a low compressive strength. An excessive amount of subsidence may result in well casing failure or rig collapse. Further, excessive compaction can permanently damage the permeability and hydrocarbon producing capability of a subterranean formation if the interstatial pores are irreparably closed. It is therefore desirable to monitor the subterranean formation to detect the onset of subsidence and compaction.
One method of monitoring formation subsidence and compaction involves implanting radioactive bullets within the subterranean formation as depth markers. The positions of the implanted bullets can then be monitored over the years using a downhole logging tool which detects the gamma rays emitted by the radioactive bullets.
One common method of implanting radioactive marker bullets into a subterranean formation involves lowering a radioactive marker bullet gun into the borehole and firing radioactive marker bullets radially out from the radioactive marker bullet gun and into the formation. When the subterranean formation is formed of a high density rock, bullets fired from the radioactive marker bullet gun may bounce off the subterranean formation and can be lost in the borehole. Some of these xe2x80x9cstrayxe2x80x9d radioactive marker bullets in the borehole may be partially lodged in the wall of the borehole or may be resting on ledges of the borehole wall. Other of the stray radioactive marker bullets may simply fall to the bottom of the borehole. These stray radioactive marker bullets are typically left in the borehole and cemented. When the next section of the borehole is drilled, some of the stray radioactive marker bullets may be crushed, thereby exposing the drilling mud to radioactive particles, and risking human exposure to radiation.
It is, therefore, an object of the present invention to provide a downhole fishing tool for retrieving stray radioactive marker bullets from a borehole during placement of radioactive marker bullets in a subterranean formation.
A further object of the invention is to provide a downhole tool assembly for implanting radioactive marker bullets into a subterranean formation and retrieving any stray radioactive marker bullets from the borehole.
A still further object of the invention is to provide a wellbore comprising a borehole, a stray radioactive marker bullet located in the borehole, and a downhole fishing tool for retrieving the stray radioactive marker bullet from the borehole.
Another object of the invention is to provide a method of retrieving stray radioactive marker bullets that does not require a fishing tool to be separately run down the borehole after the radioactive marker bullets are implanted into the subterranean formation.
It should be noted that the above-listed objects are exemplary, and not all of the above objects need to be accomplished by the invention described and claimed herein.
Accordingly, in one embodiment of the present invention, there is provided a downhole fishing tool comprising a main body and a side magnet. The main body extends along a longitudinal axis and presents a pair of axially spaced wide outer surfaces. The side magnet is coupled to the body and axially positioned between the wide outer surfaces. The side magnet presents a generally radially facing side surface. The wide outer surfaces are radially spaced from the longitudinal axis by a first maximum radial distance, while the side surface is radially spaced from the longitudinal axis by a second maximum radial distance. The first maximum radial distance is at least 10 percent greater than the second maximum radial distance.
In another embodiment of the present invention, there is provided a downhole fishing tool for retrieving radioactive marker bullets from a borehole extending into a subterranean formation. The downhole fishing tool comprises a main body, a first group of magnets, a second group of magnets, and a third group of magnets. The main body extends along a longitudinal axis and includes axially spaced proximal and distal ends. The body presents a first wide outer surface proximate the distal end, a third wide outer surface proximate the proximal end, and a second wide outer surface axially spaced from and positioned between the first and second wide outer surfaces. The first group of magnets is coupled to the body, circumferentially spaced from one another, and axially positioned between the first and second wide outer surfaces. The second group of magnets is coupled to the body, circumferentially spaced from one another, and axially positioned between the second and third wide outer surfaces. The magnets in the third group of magnets are coupled to the distal end oft he body and are spaced-apart from one another.
In still another embodiment of the present invention, there is provided an elongated downhole tool for inserting radioactive marker bullets into a subterranean formation and retrieving stray radioactive marker bullets not properly penetrating into the formation. The downhole tool comprises a radioactive marker bullet gun having a normally upper end and a normally lower end and a magnetic fishing tool having a proximal end coupled to the normally lower end of the radioactive marker bullet gun. The fishing tool includes a side magnet axially positioned between the proximal and distal ends of the body, and an end magnet positioned proximate the distal end of the tool. The side magnet presents an exposed side surface facing in a first direction and the end magnet presents an exposed bottom surface facing in a second direction. The first direction is within 30 degrees of perpendicular to the direction of extension of the longitudinal axis of the downhole tool. The second direction is within 30 degrees of parallel to the direction of extension of the longitudinal axis.
In a further embodiment of the present invention, there is provided a wellbore system in a subterranean formation comprising a borehole, a stray radioactive marker bullet, and a fishing tool. The borehole extends downwardly into the subterranean formation. The stray radioactive marker bullet is loosely positioned in the borehole. The fishing tool is positioned in the borehole and includes a plurality of magnets operable to attract and hold the stray radioactive marker bullet so that the stray radioactive marker bullet can be retrieved from the borehole along with the fishing tool.
In still a further embodiment of the present invention, there is provided a process comprising the steps of: (a) lowering a fishing tool into a borehole in a subterranean formation; (b) traversing the borehole with the fishing tool, thereby causing the fishing tool to be positioned proximate a stray radioactive marker bullet located in the borehole; (c) coupling the stray radioactive marker bullet to the fishing tool by magnetic attraction; and (d) retrieving the fishing tool and the radioactive marker bullet coupled thereto from the borehole.